The present invention relates, in general, to the field of systems and methods for monitoring vehicular traffic. More particularly, the present invention relates to a system and method for monitoring vehicular traffic with a laser rangefinding and speed measurement device utilizing a shaped divergent laser beam pattern.
Police have been using radar and laser speed measurement devices to determine vehicle speed in traffic enforcement operations for many years now. With respect to radar based devices, they generally function such that a microwave signal is emitted toward a moving vehicle and a reflection from the target returned to the device which then uses the determined Doppler shift in the return signal to determine the vehicle's speed. Radar based devices have an advantage over laser based speed guns in that they emit a very broad signal cone of energy and do not therefore, require precise aiming at the target vehicle. As such, they are well suited for fixed and mobile applications while requiring little, if any, manual operator aiming of the device. However, the broad beam of energy they emit precludes their ability to readily distinguish between adjacent vehicles on a multi-lane roadway
On the other hand, laser based speed guns employ the emission of a series of short pulses comprising a very narrow beam of monochromatic laser energy and then measure the flight time of the pulses from the device to the target vehicle and back. These laser pulses travel at the speed of light which is on the order of 984,000,00 ft./sec. or approximately 30 cm/nsec. Laser based devices then very accurately determine the time from when a particular pulse was emitted until the reflection of that pulse is returned from the target vehicle and divide it by two to determine the distance to the vehicle. By emitting a series of pulses and determining the change in distance between samples, the speed of the vehicle can be determined very quickly and with great accuracy.
While laser diodes themselves emit a somewhat divergent beam, laser rangefinders and speed guns have collimated that beam using an aspheric lens to reduce the beam width to on the order of 2 to 3 milliradians. Because of the narrow beam width employed, such devices have heretofore been predominantly relegated to hand held units which must be manually aimed at a specific target vehicle. That being the case, they have not been able to be employed in autonomous applications wherein an operator is not manually aiming the device.
In conventional fixed and semi-fixed uses of laser based speed detection devices, such as overpass mounted applications, it is important that the laser pulses be directed to a single point on an approaching target vehicle inasmuch as the frontal surface angles can vary between, for example, that of the grille (θ1) and the windshield (θ2) Where the distance to the target vehicle as measured by the laser based device is a distance M at an angle φ and the true distance to the target is D, D is then equal to M*(COS φ+SIN φ/TAN(θ1 or θ2)).
Thus, the true distance D can vary, and hence the calculated speed of the target vehicle. Normally, the angle φ is less than 10° and COS φ is then almost 1. This can reduce the calculated speed of the target vehicle, in effect giving a 1% to 2% detected speed advantage to the target vehicle as indicated below with respect to the “cosine effect”. However, the cosine effect can be minimized if an accurate tracking trajectory is maintained. On the other hand, it should be noted that the value of SIN φ/TAN(θ1 or θ2) can be greater than a normally acceptable error margin (e.g., 0.025 (2.5%)) and an even larger error can be encountered if the laser pulses are not consistently aimed at a single point on the target vehicle. As used herein, the SIN φ/TAN(θ1 or θ2) portion of the equation is referred to as a geometric error.
Both radar and laser based speed measurement devices can be used to measure the relative speed of approaching and receding vehicles from both fixed and mobile platforms. If the target vehicle is traveling directly (i.e. on a collision course) toward the device, the relative speed detected is the actual speed of the target. However, as is most frequently the case, if the vehicle is not traveling directly toward (or away from) the device but at an angle (α), the relative speed of the target with respect to that determined by the device will be slightly lower than its actual speed. This phenomenon is known as the previously mentioned cosine effect because the measured speed is directly related to the cosine of the angle between the speed detection device and the vehicle direction of travel. The greater the angle, the greater the speed error and the lower the measured speed. On the other hand, the closer the angle (α) is to 0°, the closer the measured speed is to actual target vehicle speed.